Focus-position compensator

ABSTRACT

The present invention is directed towards a focus-position compensator for reducing focus variations on a microlens array. The focus-position compensator comprises a plurality of tiles that are affixed to a structure disposed between the lenslets of the microlens array and the target of the collimated light from the lenslets. Each tile refractive index and tile thickness is chosen to obtain a tile focus-position correction that will apply to a region of the microlens array.

FIELD OF THE INVENTION

[0001] This invention relates to focus-position compensators formicrolens arrays. More particularly, it relates to using tiles to affectthe focus-position compensation.

BACKGROUND OF THE INVENTION

[0002] Associated with the information revolution is a need to increaseby many orders of magnitude the rate of information transfer. Thisrevolution is enabled by the switch from copper wire to optical fiber.Efficient implementation of this change requires optical switches tomove data from one fiber to another. For a large number of input and alarge number of output fibers, this switch is typically referred to asan optical crossbar switch.

[0003] A typical component of an optical crossbar switch is a fiberarray coupled to a microlens array in such a way that an array ofsubstantially collimated and parallel beams leave the assembly. Aschematic of a microlens array is illustrated in FIG. 1. Each microlensarray 100 is comprised of a plurality of lenslets 110. In the typicalcase, each optical fiber is associated with a single lenslet 110.

[0004] A one-to-one mapping exists between fibers and optical beamsleaving the assembly. The system performance is enhanced if each opticalbeam is substantially focused on the end of its respective opticalfiber. The construction of such a system is simplified if all of thebeams focus through the microlens array at substantially the samedistance. In such a case, the ends of all the optical fibers arearranged on a plane that is a uniform distance from the microlens array.This requires that the microlens array have a high degree of uniformitywith respect to the distance at which each lenslet focuses.

[0005] Manufacturing a microlens array with sufficiently high uniformitywith respect to the focus distance is expensive. Most often, the problemis associated with variations in the focal length of the individuallenslets. However, for the purposes of this patent document, variationsin the focus or focus distance can be due to focal-length variations ofthe lenslets or any other source of nonuniformity. In more affordablemicrolens arrays the focus distance tends to vary slowly across thearray. A typical variation is illustrated in FIG. 2. For this particularmicrolens array, the low regions 120 indicate portions of the microlensarray for which the focus distance is as much as 3% less than thenominal value. The high regions 130 indicate portions of the microlensarray for which the focus distance is as much as 4% greater than thenominal value.

[0006] To reduce the cost of an optical crossbar switch and maintainsatisfactory performance, a means for compensating for the microlensfocus variations needs to be developed.

SUMMARY OF THE INVENTION

[0007] Embodiments of the invention include a variety of focus-positioncompensators for reducing the focus variations of a microlens array. Forthe purposes of this application, reducing the focus variations is to beinterpreted broadly. The reduction in variations can be associated withreduced maximum variations, reduced mean-square variations, reducedroot-mean square variations, or some other rational measure of focusvariations.

[0008] Focus-position compensators of the present invention include aplurality of tiles. Each tile has its index of refraction and itsthickness chosen to obtain a specified tile focus-position adjustment orcorrection. The tiles are disposed in relation to the microlens arraysuch that the effects of focus variations of the microlens array arereduced.

[0009] The invention also includes methods for making focus-positioncompensators for a microlens array. To practice the method, the spatialvariation of focus distances of the microlens array is determined. Toreduce the spatial variation of the focus distances to within a desiredlimit, tiles are placed in the light path between the microlens arrayand optical fibers. The number of tiles, the spatial distribution oftiles, and the tile focus-position corrections are chosen. For eachtile, the tile focus-position correction is a function of the tilethickness and the tile refractive index; hence these properties areselected for each tile.

[0010] A reference thickness is chosen that is greater than or equal tothe maximum of all the tile thicknesses. Spacer-block thicknesses aredetermined for all the tiles. The spacer-block thickness is equal to thedifference between the reference thickness and the tile thickness. Thetiles are constructed, each having its specified thickness andrefractive index. All spacer blocks with non-zero spacer-block thicknessare constructed.

[0011] A tile tray having a receptacle for receiving each tile ismicromachined. The receptacles are positioned so that when populatedwith tiles, each tile will be properly situated relative to the othertiles.

[0012] The spacer blocks and tiles are placed in their receptacles. Ifthe corresponding spacer block exists (i.e., the spacer block hasnon-zero thickness) then the tile is placed on top of the spacer block.For tiles that don't have a corresponding spacer block, the tile issimply placed into its receptacle.

[0013] A curable bonding material is placed on top of each tile. Anintervening structure is placed on the curable bonding material. Theintervening structure can be the substrate of the microlens array, afiber-block window attached to optical fibers, or a window otherwisedisposed between the optical fibers and the microlens array. The bondingmaterial is then cured, securing the tiles to the intervening structure.The tile tray and spacer blocks are then removed.

[0014] In lieu of the curable bonding material, adhesive free bonding orfusion bonding may be used to bond the tiles to the interveningstructure.

[0015] Additional features and advantages of the invention will be setforth in part in the description that follows, and in part will beobvious from the description, or may be learned by practice of theinvention. Various embodiments of the invention do not necessarilyinclude all of the stated features or achieve all of the statedadvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings illustrate a complete embodiment of theinvention according to the best modes so far devised for the practicalapplication of the principles thereof, and in which:

[0017]FIG. 1 shows a microlens array.

[0018]FIG. 2 illustrates a typical focal length variation inducedfocus-distance variation across a microlens array.

[0019] FIGS. 3A-B show contour plots of the focus distance before andafter application of tiles. FIG. 3A shows the example situation prior tothe application of tiles. FIG. 3B shows the same case after theapplication of tiles.

[0020] FIGS. 4A-C show different embodiments of the invention. In FIG.4A, the tiles are affixed to the substrate of the microlens array. InFIG. 4B, the tiles are affixed to a window between the fiber array andthe microlens array. In FIG. 4C, the tiles are affixed to thefiber-block window.

[0021] FIGS. 5A-D illustrate aspects of some steps in making afocus-position compensator according to the invention. FIG. 5A is anexample microlens array. FIG. 5B shows a tile tray. FIG. 5C showsdifferent thickness tiles in the receptacles of the tile tray. FIG. 5Dshows the microlens array with the focus-position compensator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0022] Referring now to the drawings, where similar elements arenumbered the same, FIG. 1 depicts a microlens array 100 comprised oflenslets 110 (only one of which is labeled). As shown in FIG. 2, thefocus distance of the microlens array is typically not uniform. However.in many instances the focus distance varies slowly across the microlensarray, hence regionally compensating for focus variations cansignificantly reduce the variation. The present invention employs theuse of tiles to produce regional focus-position compensations.

[0023] Preferred embodiments of a focus-position compensator include aplurality of tiles. Each tile has a tile refractive index n_(i) and atile thickness t_(i) where i is a unique designator for each tile. Thetile refractive index for each tile is substantially uniform over thetile. Similarly, the tile thickness for each tile is substantiallyuniform over the tile. The tile refractive index and the tile thicknessare chosen to obtain a tile focus-position correction Δ_(i). Because ofthe substantial uniformity of the tile refractive index and the tilethickness over the tile, the tile focus-position correction is itselfsubstantially uniform over the tile.

[0024] The tile focus-position correction is chosen to reduce theeffects of the focus variation of the microlens array. Once the focusvariation of the microlens array is known, appropriate focus distanceincreases can be mapped to each region of the microlens array, with atile corresponding to each region. The tile is disposed in relation tothe microlens array such that the effects of focus variations of themicrolens array are reduced. The spatial relationship between the tilesand the microlens array should ensure that the tiles are in the lightpath between the lenslets and the target of the substantially collimatedlight.

[0025] In most instances, the microlens array is rectangular; hencetiles having a rectangular planform are preferred. However, the broadscope of the invention is intended to include microlens arrays that arenot rectangular and also tiles that do not have rectangular planforms,even if the microlens array is rectangular.

[0026] A simple example is illustrated in FIGS. 3A-B. FIG. 3A showscontours of the focus distance of a microlens array. For simplicity, thefocus distance is shown as a continuous variable. In reality, eachlenslet has its own focus distance, so focus distance only has meaningat the discrete lenslets. However, with a dense microlens array, wherethe focus distance varies slowly from lenslet to lenslet, a continuousfunction is a good approximation to the actual situation. In thisexample, the focus distance varies according to the equation f=4.8mm+0.02(0.5 x+y), where x and y are measured in mm. This means that atposition x=0, y=0, the focus distance is 4.8 mm but as x and y increase,the focus distance increases linearly. In the middle of the microlensarray at x=10 mm and y=5 mm, the focus distance is 4.8 mm+0.2 mm=5.0 mm.At the upper right edge of the microlens array at x=20 mm and y=10 mm,the focus distance is 4.8 mm+0.4 mm=5.2 mm. Therefore the focus distancevaries 4% higher and lower than its central value of 5.0 mm.

[0027] Suppose that four tiles are chosen to correct the effects of themicrolens focus variation. For simplicity, each tile will be rectangularand will have nominal dimensions of 10 mm in the x direction and 5 mm inthe y direction. The tiles are evenly spaced on the microlens array asshown in FIG. 3B. As discussed later, in practice, gaps will probablyexist between each tile. Even if the tiles are assembled so that no gapexists, the focus-position correction can be discontinuous at the tileboundaries; hence the tile boundaries are preferably aligned withboundaries between lenslets. In this example, a focus-positioncorrection of 0.3 mm is used for the lower-left tile; 0.2 mm for the topleft and lower right tiles; and 0.1 mm for the tile at the top right.The focus position for light incident at the center of each tile is nowcorrected in such a way that the maximum focus variation is only 0.1 mm,or slightly less than 2%, similar to a microlens array with a focallength of 5.2±0.1 mm.

[0028] Alternatively, a focus-position correction of 0.2 mm could beused for the lower-left tile; 0.1 mm for the top left and lower righttiles; and no correction for the tile at the top right. The focusposition for light incident at the center of each tile would now besimilar to a microlens array with a focal length of 5.1 mm and themaximum variation would remain 0.1 mm. Whether the original oralternative correction scheme is used will depend upon factors specificto each application of the invention. Both correction schemes areconsidered to be within the scope of the invention.

[0029] The focus-position corrections can be achieved by adjusting thetile thickness, the tile refractive index, or both. In most cases, thefocus-position correction is related to these parameters by theequation: Δ_(i)=t_(i)(1−n_(media)/n_(i)), where n_(media) is therefractive index of the media that the tile is supplanting. For mostcases of practical interest, the media is air, some inert atmosphere, ora vacuum. Generally, n_(media) can be approximated as 1 in those cases.Other appropriate relationships between the focus-position correction,the tile thickness, the tile refractive index, and the index ofrefraction of the external media, as well as other relevant parametersmay be used when required by the situation.

[0030] In the original scheme of the previous example, takingn_(media)=1, all of the tiles could have been made with thickness 0.6mm. In this case, to achieve a 0.3 mm focus-position correction, thetile in the lower left would require a refractive index of 2. The tilesin the upper left and lower right would require a refractive index of1.5 to achieve a 0.2 mm focus-position correction. To achieve a 0.1 mmfocus-position correction, the tile in the upper right would require arefractive index of 1.2.

[0031] Alternatively, in the previous example with n_(media)=1, all ofthe tiles could have been made of a material having a refractive indexof 1.5. In this case, the tile in the lower left would need to be 0.9 mmthick, the upper left and lower right tiles would need to be 0.6 mmthick, and the upper right tile would be 0.3 mm thick. Clearly,combinations of different tile refractive indices and thicknesses can beused to achieve the desired focus corrections.

[0032] A large number of materials may be used to obtain the desiredproperties of the tiles. Clearly index of refraction is important, butuniformity of the material, cost, ease with which it can be machined andhandled, scratch resistance, etc. are just some of the other materialproperties that might influence which material is best suited for aspecific tiling application. Some materials that are believed to beuseful for tiles include: glass, sapphire, fused silica, calcite,quartz, Calcium Fluoride, Magnesium Fluoride, Zinc Selenide, ZincSulfide, Germanium, Silicon, Gallium Arsenide, Gallium Phosphide,Aluminum Gallium Arsenide, Indium Gallium Arsenide, and KRS5 (aninfrared window material that contains Tallium Bromide Thalium Iodide).

[0033] The tiles may be affixed to a variety of intervening structuresto effect the focus correction. The term intervening structure is usedbecause the structure is placed between the lenslets and the target forthe collimated light. FIGS. 4A-C show side views of three differentintervening structures.

[0034] In the embodiment illustrated in FIG. 4A, each lenslet 110 of themicrolens array 100 is supported in a substrate 150. The light from eachlenslet 110 is to be focused to a corresponding optical fiber 170, whichis supported in a fiber block fiber holder 210 and covered by a fiberblock window 180. The fiber block fiber holder 210 comprises one or moreelements that support the optical fibers 170. The fiber block fiberholder 210 includes any appropriate means for supporting the opticalfibers that is known to those skilled in the art. The plurality ofoptical fibers 170 attached to the fiber-block fiber holder 210 andcovered by the fiber block window 180 is known as the fiber array 160.In this embodiment, each tile 200 is affixed to the substrate 150 of themicrolens array 100. Although the tiles 200 are shown as having variablethickness, as discussed earlier, they may instead and/or also havevariable refractive indices to achieve the desired focus correction.Note that to avoid clutter in the figure, although many lenslets,optical fibers, and tiles are shown, only one of each is explicitlylabeled.

[0035]FIG. 4B illustrates an embodiment in which a window 190 serves asthe intervening structure to which the tiles 200 are affixed. FIG. 4Cshows an embodiment in which the tiles 200 are affixed to thefiber-block window 180 of the fiber array 160. Other embodiments thatfall within the broad scope of the claims are also considered as part ofthe invention. For instance, some of the tiles can be affixed to thesubstrate 150 and others to the fiber-block window 180.

[0036] A variety of fastening techniques may be employed to affix thetiles to the intervening structure. In some preferred embodiments abonding material is used. In the most preferred of these embodiments acurable bonding material is used. Most preferably, UV (ultraviolet)curing cement is used to affix the tiles to the intervening structure.Alternatively, adhesive free bonding (of which fusion bonding is aspecific example) may also be used to affix the tiles to the interveningstructure.

[0037] Because of the typically large difference in refractive indexbetween the tiles and the external media, an antireflection coating isoften added to the side of the tiles that is not affixed to theintervening structure, i.e., the side exposed to the external media.Refractive index variations between the tiles and the interveningstructure are typically not so large, hence antireflection coatings arenot usually employed at that interface.

[0038] The construction of focus-position compensators corresponding tothe present invention can be accomplished by separately bonding eachtile in its proper position. However, the invention also includes moreefficient methods for making focus-position compensators for a microlensarray.

[0039] To practice these methods, the spatial variation of the focusdistance for the microlens array needs to be determined. One way todetermine the focal length variation is with a Zygo Microlupi. A ZygoMicrolupi is an instrument that can measure the exact curvature of eachlenslet in a microlens array. From the curvature, the focal length ofthe individual lenslets is calculated using techniques known to thoseskilled in the art. Any additional alterations to the focus distanceassociated with each lenslet can then be added to the computed focallength. Knowledge of the focus distance of each lenslet is used todetermine the spatial variation of the microlens focus distances. Otherapproaches for determining the focus variation can also be usedincluding, for example, a Hartmann sensor.

[0040] The number of tiles, the spatial distribution of tiles, and thetile focus-position correction Δ_(i) of the i-th tile are chosen suchthat the focus variation of the microlens array is reduced to within adesired limit. The best choice for all of the parameters depends uponthe details of each case.

[0041] The tile thickness t_(i) and tile refractive index n_(i) areselected to achieve the desired tile focus correction Δ_(i). Asdiscussed earlier, a useful formula that relates the focus correction,the thickness and the refractive index isΔ_(i)=t_(i)(1−n_(media)/n_(i)), where n_(media) is the refractive indexof the media that the tile supplants. Other relationships may be used asappropriate to determine the best choice of parameters for any givensituation.

[0042] After all the tile thicknesses are known, a reference thicknesst_(ref) is chosen. The reference thickness should be greater than orequal to the maximum tile thickness. The reference thickness is used todetermine spacer-block thicknesses. The thickness of each spacer blockis designated s_(i) and is determined by s_(i)=t_(ref)−t_(i).

[0043] The tile and spacer block thicknesses are constructed to havetheir appropriate shapes and sizes. To reduce costs, in preferredembodiments, the tiles are cut from sheets of material that are obtainedin the desired thicknesses. The sheets may be of any appropriate tilematerial. Silica or glass is used in the most preferred embodimentsalthough, as discussed earlier, many other materials are suitable. Thespacer blocks can be similarly constructed. If any of the spacer blockshas zero thickness (i.e., s_(i)=0) then that spacer block is notnecessary.

[0044] A tile tray having a receptacle for receiving each tile needs tobe micromachined. Each of the receptacles is positioned to receive itscorresponding tile. In preferred embodiments the tile tray isconstructed of Silicon and is micromachined with a deep reactive ionetching (DRIE) process. The use of the tile tray greatly facilitatestile alignment, especially when many tiles are used.

[0045] All spacer blocks having finite thickness (i.e., s_(i)>0) areplaced in their respective receptacles. Each tile is placed on top ofits corresponding spacer block where one exists (i.e., s_(i)>0). Fortiles with no corresponding spacer block, the tiles are just placed intheir respective receptacles. Note that because of the way in which thespacer block thicknesses are determined, the sum of the tile thicknessand the spacer-block thickness is always the same and equals t_(ref).This means that the top of all the tiles are at the same height.

[0046] The desired intervening structure is placed on top of the tiles,aligned and bonded. The order in which this is done depends, in part,upon the type of intervening structure. As discussed earlier, inpreferred embodiments the intervening structure is a substrate of themicrolens array, a fiber-block window, or a window placed between themicrolens array and a plurality of optical fibers.

[0047] In the case in which the intervening structure is the substrateof the microlens array or a fiber-block window, the tile tray is alignedafter the intervening structure is placed on top of the tiles but priorto bonding. In the case in which the intervening structure is thesubstrate of the microlens array, the tile tray is usually aligned withthe microlens array using X—Y positioners and a rotation stage to setthe correct clocking angle. The alignment is most easily accomplishedunder a microscope. In preferred embodiments the alignment process triesto ensure that the tile boundaries occur between the lenslets, so thatthe light from a lenslet does not pass close enough to the tile boundaryto become significantly distorted. A similar alignment is performed inthe case in which the intervening structure is the fiber-block window.In this case, the tile tray is aligned with the optical fibers, againtrying to ensure that a minimum amount of light is distorted by the tileboundaries. In preferred embodiments, the alignment is accurate to about0.01 mm.

[0048] Any appropriate bonding process known to those skilled in the artmay be used. Some preferred embodiments employ adhesive free bonding,including, fusion bonding. Other preferred embodiments employ a curablebonding material, most preferably, UV curing cement.

[0049] In cases in which a curable bonding material is used, the curablebonding material is placed on top of each tile and then the desiredintervening structure is placed on top of the curable bonding material.In the case of UV curing cement, the curing involves exposure toultraviolet radiation.

[0050] After the tiles are bonded, the tile tray and the spacer blocksare removed. The tiles are now affixed to the intervening structure. Incases in which the intervening structure is a window disposed betweenthe microlens array and the optical fibers, the window must be alignedwith the microlens array and the optical fibers. This is typically, butnot necessarily done after the tiles are bonded to the interveningstructure.

[0051] As a detailed example, consider a microlens array containing 1200lenslets arranged in 30 rows of 40 lenslets each. Except that somewhatlarger and fewer lenslets are shown FIG. 5A is illustrative of theexample microlens array 100. Only a single lenslet 110 is labeled toreduce clutter in the figure. The lenslets 110 in this example arearranged with a 1 -mm pitch, meaning that the lenslet centers areseparated by 1 mm in each row and each column. Each lenslet 110 isapproximately 0.95 mm in diameter, so they are separated from theirnearest neighbors by approximately 0.1 mm. A 2.5-mm border 140 extendsaround the edges of the microlens array 100 in this example.

[0052] The spatial variation of the microlens array focus distance isdetermined. Four tiles, each with a refractive index of approximately1.5 are to be used. Each tile has a rectangular planform with nominaldimensions of 15 mm by 20 mm. Three different tile thicknesses are to beused 0.129 mm, 0.258 mm, and 0.516 mm. These will produce focus-positioncorrections of 0.043 mm, 0.086 mm, and 0.172 mm, respectively. In thiscase the reference thickness is chosen as 0.516 mm; hence only threespacer blocks are required, two having a thickness of 0.258 mm and onehaving a thickness of 0.387 mm. Because one of the tiles has a thicknessequal to the reference thickness, the corresponding spacer-blockthickness is zero; therefore no corresponding spacer block is used.Although the thicknesses are defined to three significant figures,deviations in the thicknesses of as much as ±0.01 mm have beendetermined to be acceptable in this example.

[0053] Although the tiles are nominally 15 mm by 20 mm, to adequatelysupport them in a tile tray requires that they be somewhat smaller. Inthis example, the tiles and the spacer blocks are cut to 14.925 mm by19.925 mm with acceptable errors of ±0.015 mm. Note that this impliesthat the tiles will be separated by a gap. Providing that the collimatedlight beam remains sufficiently far from the edge of the tile, the gapsare not problematical.

[0054]FIG. 5B shows a schematic of the tile tray 300. The startingmaterial for the tile tray is a piece of Silicon 35 mm by 45 mm by 1.5mm. Four rectangular receptacles 320 are micromachined in the Siliconusing DRIE. These regions are 14.950 mm by 19.950 mm with a tolerance of±0.005 mm. The receptacles are 0.450 mm deep. A 2.5-mm border 310 ismaintained around the edge of the tile tray 300 to conform to the border140 around the edge of the microlens array 110 (as seen in FIG. 5A).

[0055] The three spacer blocks are inserted into their respectivereceptacles in the tile tray. Note that each one lies at the bottom ofits receptacle. The four tiles are placed into their respectivereceptacles. Three of the tiles overlie spacer blocks; the 0.516 mmthick tile lies at the bottom of its receptacle. FIG. 5C shows the tiles200 in the tile tray 300.

[0056] In this example, UV curing cement (for example, Norland 61) isplaced on top of the tiles. The substrate of the microlens array isplaced on top of the tiles. Under a microscope, using X—Y positionersand a rotation stage to set the correct clocking angle, the tile tray isaligned with the microlens array to within 0.01 mm.

[0057] After alignment, ultraviolet radiation is used to cure thecement. The tile tray and spacer blocks are then removed. The finishedproduct is schematically shown in FIG. 5D. The tiles 200 overlay thesubstrate (transparent) of the microlens array 100. The lenslets 110 canbe seen through the tiles 200.

[0058] The above description and drawings are only illustrative ofpreferred embodiments, and the present invention is not intended to belimited thereto. Any modification of the present invention that comeswithin the spirit and scope of the following claims is considered partof the present invention.

What is claimed is:
 1. A focus-position compensator for reducing focusvariations on a microlens array, the focus-position compensatorcomprising: a plurality of tiles, each tile having a tile refractiveindex and a tile thickness, the tile refractive index for each tilebeing substantially uniform over the tile, and the tile thickness beingsubstantially uniform over the tile, the tile refractive index and thetile thickness being chosen to obtain a tile focus-position correction,at least two of the tiles having different tile focus-positioncorrections, the plurality of tiles being disposed in relation to themicrolens array such that effects of focus variations of the microlensarray are reduced.
 2. The focus-position compensator, according to claim1, wherein any of the tiles is made from: glass, sapphire, fused silica,calcite, quartz, Calcium Fluoride, Magnesium Fluoride, Zinc Selenide,Zinc Sulfide, Germanium, Silicon, Gallium Arsenide, Gallium Phosphide,Aluminum Gallium Arsenide, Indium Gallium Arsenide, or KRS5.
 3. Thefocus-position compensator, according to claim 1, wherein the pluralityof tiles are affixed to a window disposed between the microlefts arrayand a plurality of optical fibers.
 4. The focus-position compensator,according to claim 3, wherein any of the tiles is made from: glass,sapphire, fused silica, calcite, quartz, Calcium Fluoride, MagnesiumFluoride, Zinc Selenide, Zinc Sulfide, Germanium, Silicon, GalliumArsenide, Gallium Phosphide, Aluminum Gallium Arsenide, Indium GalliumArsenide, or KRS5.
 5. The focus-position compensator, according to claim1, wherein the plurality of tiles are affixed to the microlens array. 6.The focus-position compensator, according to claim 5, wherein each tilehas a rectangular planform.
 7. The focus-position compensator, accordingto claim 6, wherein the tiles are attached to the microlens array byadhesive free bonding.
 8. The focus-position compensator, according toclaim 6, wherein the tiles are attached to the microlens array with UVcuring cement.
 9. The focus-position compensator, according to claim 8,wherein all the tiles have approximately the same index of refraction.10. The focus-position compensator, according to claim 5, wherein any ofthe tiles is made from: glass, sapphire, fused silica, calcite, quartz,Calcium Fluoride, Magnesium Fluoride, Zinc Selenide, Zinc Sulfide,Germanium, Silicon, Gallium Arsenide, Gallium Phosphide, AluminumGallium Arsenide, Indium Gallium Arsenide, or KRS5.
 11. Thefocus-position compensator, according to claim 1, wherein the pluralityof tiles are affixed to a fiber-block window connected to the pluralityof optical fibers.
 12. The focus-position compensator, according toclaim 11, wherein the fiber-block window is comprised of glass orsilica.
 13. The focus-position compensator, according to claim 11,wherein any of the tiles is made from: glass, sapphire, fused silica,calcite, quartz, Calcium Fluoride. Magnesium Fluoride, Zinc Selenide,Zinc Sulfide, Germanium, Silicon, Gallium Arsenide, Gallium Phosphide,Aluminum Gallium Arsenide, Indium Gallium Arsenide, or KRS5.
 14. Amethod of making a focus-position compensator for a microlens arrayhaving a substrate, the method comprising the steps of: determining thespatial variation of focus distance of the microlens array; choosing anumber of tiles, a spatial distribution of tiles, and a tilefocus-position correction Δ_(i) of the i-th tile, such that the focusvariation of the microlens array is reduced to within a desired limit;selecting a tile thickness t_(i) of the i-th tile and a tile refractiveindex n_(i), so as to obtain the chosen tile focus-position correctionΔ_(i), choosing a reference thickness t_(ref), the reference thicknessbeing greater than or equal to the maximum tile thickness; determining aspacer-block thickness si corresponding to the i-th tile such thats_(i)=t_(ref)−t_(i); constructing tiles, the i-th tile having thicknesst_(i); constructing spacer blocks for all s_(i)>0, the i-th spacer blockhaving thickness s_(i); micromachining a tile tray, the tile tray havinga receptacle for receiving each tile, the i-th receptacle beingpositioned to receive the i-th tile; for s_(i)>0, placing the i-thspacer block in the i-th receptacle; placing the i-th tile in the i-threceptacle, wherein for s_(i)>0, i-th tile is disposed on top of thei-th spacer block; placing an intervening structure on top of the tiles;bonding the tiles to the intervening structure; removing the tile trayand the spacer blocks.
 15. The method, according to claim 14, wherein:the bonding is adhesive free bonding.
 16. The method, according to claim14, wherein: the bonding comprises: placing a curable bonding materialon top of each tile prior to placing the intervening structure on top ofthe tiles; and curing the curable bonding material after placing theintervening structure on top of the tiles.
 17. The method, according toclaim 16, wherein: the intervening structure is a window placed betweenthe microlens array and a plurality of optical fibers; and furthercomprising the step of: aligning the window with the microlens array.18. The method, according to claim 16, wherein: the interveningstructure is a fiber-block window connected to a plurality of opticalfibers; and further comprising the step of: aligning the tile tray withthe optical fibers prior to curing the curable bonding material.
 19. Themethod, according to claim 16, wherein: the intervening structure is thesubstrate of the microlens array; and further comprising the step of:aligning the tile tray with the microlens array prior to curing thecurable bonding material.
 20. The method, according to claim 19, whereineach tile is made from glass.
 21. The method, according to claim 19,wherein each tile is made from silica.
 22. The method, according toclaim 19, wherein the tile tray is rectangular and the receptacles arerectangular and evenly spaced.
 23. The method, according to claim 19,wherein the tile tray is made from Silicon and the micromachining isdone with Deep Reactive Ion Etching (DRIE).
 24. The method, according toclaim 19, wherein the curable bonding material is UV curing cement. 25.The method, according to claim 19, wherein a microscope and X—Ypositioners and a rotation stage are used to align the tile tray and themicrolens array.
 26. The method, according to claim 25, wherein the tiletray and the microlens array are aligned to within 10 μm.